Dynamic solid state diffractive optics applied for reading a diffractive optics memory

ABSTRACT

The apparatus and method of the present invention employ solid state dynamic diffractive optical elements for reading information from a diffractive optics memory. The diffractive optics memory has stored therein information located at a plurality of points of the memory and at a plurality of angles at each one of the points so as to form a plurality of packets of information at each of the points. A matrix of the dynamic diffractive optical elements is configured to shape a laser beam to address the memory at one of the angles of one of the points to reconstruct one of the packets of information.

FIELD OF INVENTION

The present invention generally relates to a diffractive optics memory.In particular, the present invention relates to an apparatus for readinginformation from the diffractive optics memory using dynamic solid statediffractive optics devices.

BACKGROUND OF THE INVENTION

The large storage capacities and relative low costs of CD-ROMS and DVDshave created an even greater demand for still larger and cheaper opticalstorage media. Holographic memories have been proposed to supersede theoptical disc as a high-capacity digital storage medium. The high densityand speed of the holographic memory comes from three-dimensionalrecording and from the simultaneous readout of an entire packet of dataat one time. The principal advantages of holographic memory are a higherinformation density (10¹¹ bits per square centimeter or more), a shortrandom access time (˜100 microseconds and less), and a high informationtransmission rate (10⁹ bit/sec).

In holographic recording, a light beam from a coherent light source(e.g., a laser) is split into a reference beam and an object beam. Theobject beam is passed through a spatial light modulator (SLM) and theninto a storage medium. The SLM forms a matrix of shutter (in the binarycase) or, more generally, a matrix of photocells modulating the lightintensity that represents a packet of data. The object beam passesthrough the SLM which acts to modulate the object beam with the datainput to the SLM. The modulated object beam is then processed by anappropriate optical system and then directed to one point on the storagemedium by an addressing mechanism where it intersects with the referencebeam to create a hologram representing the packet of data.

An optical system consisting of lenses and mirrors is used to preciselydirect the optical beam encoded with the packet of data to theparticular addressed area of the storage medium. Optimum use of thecapacity of a thick storage medium is realized by spatial and angularmultiplexing. In spatial multiplexing, a set of packets is stored in thestorage medium shaped into a plane as an array of spatially separatedand regularly arranged subholograms by varying the beam direction in thex-axis and y-axis of the plane. Each subhologram is formed at a point inthe storage medium with the rectangular coordinates representing therespective packet address as recorded in the storage medium. In angularmultiplexing, recording is carried out by keeping the x- andy-coordinates the same while changing the irradiation angle of thereference beam in the storage medium. By repeatedly incrementing theirradiation angle, a plurality of packets of information is recorded asa set of subholograms at the same x- and y-spatial location.

Previous holographic devices for recording information in a highlymultiplexed volume holographic memory, and for reading the informationout, require components and dimensions having a large size which placesa limit on the ability to miniaturize these systems. Because previousholographic devices use motors and large-scale components such asmirrors and lenses, the addressing systems of these previous devices areslow. Furthermore, the mechanical components of these previous devicesneed frequent maintenance to correct errors and dysfunction coming, forinstance, from wear and friction (i.e., tribology effect). Furthermore,previous addressing systems are expensive because they use complexsystems for control. Thus, their prices cannot be lowered by massproduction. Moreover, previous devices are not economical in theirenergy consumption. Even when previous addressing devices are accuratewhen new, the wear and friction of the interacting surfaces that are inrelative motion lowers their accuracy with time.

In view of the foregoing, it would be desirable to provide one or moretechniques which overcomes the above-described inadequacies andshortcomings of the above-described proposed solutions.

Thus, it is an object of the present invention to provide a dynamicdiffractive optics reading system made of solid state components.

It is another object of the present invention to provide an apparatusfor reading a diffractive optics memory having components that operatefaster than systems produced today.

It is a further object of the present invention to provide an apparatusfor reading a diffractive optics memory having components that moreaccurately target movement of the laser beams onto the recorded regionsof the diffractive optics memory.

It is yet a further object of the present invention to provide anapparatus for reading a diffractive optics memory having miniaturecomponent sizes.

It is still another object of the present invention to provideinexpensive components for a dynamic diffractive optics reading system.

SUMMARY OF THE INVENTION

In order to achieve the above-mentioned objectives, the presentinvention comprises solid state dynamic diffractive optical elements forreading information from a diffractive optics memory. The diffractiveoptics memory has stored therein information located at a plurality ofpoints on the memory and at a plurality of angles at each one of thepoints so as to form a plurality of packets of information at each ofthe points. The diffractive optics memory is arranged in the form of amatrix, or alternately, may be arranged in other forms, such as a tapeand a disk. A matrix of the dynamic diffractive optical elements isconfigured to shape and angularly direct a wavefront of a coherent lightbeam to the memory at one of the angles of one of the points toreconstruct one of the packets of information.

In a further aspect of the present invention, a laser generates thecoherent light beam, and an acousto-optic device deflects the wavefrontof the coherent light beam toward the dynamic diffractive opticalelements at a deflection angle with respect to a plane formed by thediffractive optical elements.

In still another aspect of the present invention, the wavefront isshaped by phase shifting of the diffractive optical elements.

In yet another aspect of the present invention, each of the diffractiveoptical elements comprises a pyramidal element or a piston element.

In still another aspect of the present invention, a computer isconfigured to program the diffractive optical elements and theacousto-optic device so as to address the memory at one of the pointsand one of the angles to reconstruct one of the packets.

In a further aspect of the present invention, a detector array isconfigured comprising a plurality of cells receiving a portion of thewavefront deflected by the diffractive optical elements and deflected bythe memory. The detector array may be a CCD detector array.

In another aspect of the present invention, a low powered laser isconfigured to produce the coherent light beam.

In still another aspect of the present invention, each of the pluralityof points stores one or more of the packets of information.

The present invention thus achieves the objectives of fast access time,long life duration, miniaturization, reliability, stability, a limitedresponse to surrounding perturbations, and a lower cost through massproduction.

A more thorough disclosure of the present invention is presented in thedetailed description which follows and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present invention,reference is now made to the appended drawings. These drawings shouldnot be construed as limiting the present invention, but are intended tobe exemplary only.

FIG. 1 is a schematic representation of an apparatus for recording aninterference pattern according to the present invention.

FIG. 2 is a schematic representation of a matrix of points formed in astorage medium.

FIG. 3 illustrates the principals of a diffractive optical element.

FIGS. 4 a, 4 b shows diffraction efficiency as a function ofreconstruction wavelength for different levels of quantization.

FIG. 4 c shows a diffractive optical element at different levels ofquantization.

FIG. 5 a shows a top view of a pyramidal element.

FIG. 5 b shows a side view of a pyramidal element.

FIG. 5 c shows the principal of phase modulating by a pyramidal element.

FIG. 6 a shows a top view of a piston element.

FIG. 6 b shows a side view of a piston element.

FIG. 6 c shows the principal of phase shifting by a piston element.

FIG. 7 a shows a SEM photograph of an array of pyramidal elements.

FIG. 7 b shows a SEM photograph of an array of piston elements.

FIG. 8 shows an addressing device using diffractive optical elements.

FIG. 9 shows a dynamic reading system using diffractive optical elementsaccording to the present invention.

FIG. 10 shows an acousto-optic system according to the presentinvention.

FIG. 11 shows a reader of a diffractive optics memory according to thepresent invention.

FIG. 12 shows diffraction of a light beam from a diffraction gratingaccording to the present invention.

FIG. 13 shows a diffraction grating according to the present invention.

FIG. 14 shows orientation of a diffractive optics element according tothe present invention.

FIG. 15 shows the diffracted beam angle for different orientations of adiffractive optics element according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Recording Process

FIG. 1 shows a schematic of the important signals involved in recordinga diffraction pattern, or alternately a hologram, in a storage mediumusing angular and spatial multiplexing. Various diffractive recordingprocesses have been developed in the art and further details can befound in the book Holographic Data Storage, Springer (2000) edited by H.J. Coufal, D. Psaltis, and G. T. Sincerbox. In this specification, theterm “diffractive” is used throughout to differentiate prior artholographic technology used for 3-D image generation from diffractivetechnology necessary for the generation of a storage medium. Forexample, diffraction efficiency is critical to the viability of anymaterial to be used as a diffractive storage medium. The quality ofinterference constituting a 3D-hologram is simple to achieve compared tothe quality required to realize a storage medium. Moreover, a storagediffractive pattern can also be implemented by using other techniquesthan the interference of a reference and object beam, such as using ane-beam and a microlithography process to etch materials to generatediffractive structures. For all these reasons, the specification hereinintroduces the concept of a broader diffractive optics technology.

In forming a diffractive pattern, or alternately a hologram, a referencebeam 1 intersects with an object beam 4 to form a diffraction pattern(e.g., a sub-hologram) 8 a (referred to alternately as a point)extending through the volume of storage medium 8. There is a separatediffraction pattern at point 8 a extending through the volume for eachangle and spatial location of the reference beam 1. The object beam 4 ismodulated with a packet of information 6. The packet 6 containsinformation in the form of a plurality of bits or pixels. The source ofthe information for the packet 6 can be a computer, the Internet, or anyother information-producing source. The hologram impinges on the surface8 a of the storage medium 8 and extends through the volume of thestorage medium 8. The information for the packet 6 is modulated onto thestorage medium 8 by spatial multiplexing and angle multiplexing. Anglemultiplexing is achieved by varying the angle α of the reference beam 1with respect to the surface plane of the storage medium 8. A separatepacket 6 of information is recorded in the storage medium 8 as adiffraction pattern (e.g., a sub-hologram) for each chosen angle α andspatial location. Spatial multiplexing is achieved by shifting thereference beam 1 with respect to the surface of the storage medium 8 sothat the point 8 a shifts to another spatial location, for example point8 a′, on the surface of the storage medium 8.

The storage medium 8 is typically a three-dimensional body made up of amaterial sensitive to a spatial distribution of light energy produced byinterference of the object light beam 4 and the reference light beam 1.A diffraction pattern may be recorded in a medium as a variation ofabsorption or phase or both. The storage material must respond toincident light patterns causing a change in its optical properties. In avolume hologram, a large number of packets of data can be superimposed,so that every packet of data can be reconstructed without distortion. Avolume (thick) hologram may be regarded as a superposition of threedimensional gratings recorded in the depth of the emulsion eachsatisfying the Bragg law (i.e., a volume phase grating). The gratingplanes in a volume hologram produce change in refraction and/orabsorption.

Several materials have been considered as storage material for opticalstorage systems because of inherent advantages. These advantages includea self-developing capability, dry processing, good stability, thickemulsion, high sensitivity, and nonvolatile storage. Some materials thathave been considered for volume holograms are photorefractive crystals,photopolymer materials, and polypeptide material.

FIG. 2 shows in greater detail the storage medium 8 arranged in the formof a flat sheet, herein referred to as a matrix. In this example, thematrix is 1 cm². Each of a plurality of points on the matrix is definedby its rectilinear coordinates (x, y). An image-forming system (notshown) reduces the object beam 4 to the sub-hologram 8 a having aminimum size at one of the x, y point of the matrix. A point in physicalspace defined by its rectilinear coordinates contains a plurality ofpackets 8 b.

In this case, a 1 mm² image 8 a is obtained by focusing the object beam4 onto the storage medium 8 centered at its coordinate. Due to thisinterference between the two beams 1,4, a diffractive image 8 a 1 mm² insize is recorded in the storage material 8 centered at the coordinatesof the matrix. Spatial multiplexing is carried out by sequentiallychanging the rectilinear coordinates. The object beam 4 focuses on thestorage material 8 so that a separate image 8 a is recorded at a uniqueposition in the plane defined by its coordinates (x, y). This spatialmultiplexing results in a 10 by 10 matrix of diffractive images 8 a.Angle multiplexing is carried out by sequentially changing the angle ofthe reference beam 1 by means of mirrors (not shown). Angle multiplexingis used to create 15–20 packets of information 8 b corresponding to 15discrete variations of the angle of incidence of the reference beam.However, also achievable is angle multiplexing using 20–25 angles usingsimple angular multiplexing or 40–50 angles using symmetrical angularmultiplexing. A data packet is reconstructed by shinning the referencebeam 1 at the same angle and spatial location in which the data packedwas recorded. The portion of the reference beam 1 diffracted by thestorage material 8 forms the reconstruction, which is typically detectedby a detector array. The storage material 8 may be mechanically shiftedin order to store data packets at different points by its coordinates(x, y).

Reading Process

Diffractive optical elements (DOEs) are wavefront processors which canmodify the characteristics of laser beams. A DOE works by breaking up anincoming light wavefront into a large number of waves and thenrecombining these waves to form a completely new wavefront. FIG. 3 showsthe input waveform broken up by the DOE into diffracted beams of order+1 and order −1 symmetric about the zeroth order, where the zeroth orderis the transmitted beam without alteration of the direction.

By this process, an input wavefront is shaped by the DOE into adiffractive wavefront. A DOE can reconstruct the desired wave fronteither in near field (Fresnel element) or in far field (Fourierelement). A description of the current state of diffractive optics canbe found in the book entitled “Digital Diffractive Optics: AnIntroduction to Planar Diffractive Optics and Related Technology” by B.Kress and P. Meyruies (Wiley & Sons, 2000).

The transmittance of a DOE is expressed by the formula shown, asfollows:

$T_{H} = {A \cdot {\mathbb{e}}^{{\mathbb{i}}{({{\frac{2\pi}{\lambda}{n \cdot e}} + \phi^{\prime}})}}}$where A is the wave amplitude, Lamda (λ) is the wavelength, n is therefractive index, e is the depth of carving of the grating (featuredepth), and φ′ is the phase modulation of the DOE output wave.

There are two types of DOEs: static and dynamic. The present inventionuses dynamic diffractive optical elements (DDOE). In contrast, a staticDOE is a DOE in which the diffraction pattern (the structure that iscalled “feature” of the DOE surface) is not changing with time. Anexample of a static diffractive optical element is a Fresnel lens etchedon a substrate. A DDOE is an optical element in which the diffractionpattern is changing with time. As will be explained below, the microactuators of the mirror elements 10, 40 of FIGS. 5, 6 are powered andcontrolled by electricity which modifies the diffraction structure bychanging the features spacing and positioning of the mirrors. The mirrorelements 10, 40 are used in implementing the dynamic diffractive opticalelements of the present invention.

FIGS. 4 a, 4 b represent the diffraction efficiency for different levelof sampling and for different angles. The first order diffractionefficiency (in normalized units) is plotted versus the reconstructionwavelength (nm) for binary, four level, eight level, and the sixteenlevel structures shown in FIG. 4 c. FIG. 4 c also shows the minimumfeature size and maximum etch depth for each of the structures.

FIG. 4 c is a grating profile, that is to say that it is an enlargedview of a cut grating at different levels of quantization. The fivelevels of sampling shown are binary, quaternary, eight levels, sixteenlevels, and analog. The diffraction efficiency related to the number ofsamples to implement a phase profile, the “ideal one” being the analogprofile. The curves 11–16 represents the diffraction efficiencydepending on the number of structure samplings 4, 8, or 16. In FIGS. 4 aand 4 b curves 11–16 are shown in a graph plotting the reconstructionwavelength in nanometers against the diffraction efficiency innormalized units. Curves 11 and 14 represent the diffractive wavefrontfor the four level structure. Curves 12 and 15 represent the diffractivewavefront for the 8 level structure. Curves 13 and 16 represent thediffractive wavefront for the 16 level structure. FIG. 4 a representsthe diffraction efficiency for the first order (+1) diffracted wavefront(see FIG. 3). FIG. 4 b represents the diffraction efficiency for thefirst order (−1) diffracted wavefront. For instance for 800 nanometer inFIG. 4 a, the diffraction efficiency for the 4 level structure isroughly 0.35, for the 8 level structure it is 0.75, and roughly 0.8 forthe 16 levels structure. In FIG. 4 b, representing the output at order−1, there is at 800 nanometer the four level structure is approximately0.02, for the 8 level structure it is 0.04, and for the 16 levelstructure it is 0.4.

FIGS. 5 and 6 show two micromirror elements, a pyramidal element 10 anda piston element 40. It will be shown how in the present invention thesemicromirrors can be used to implement dynamic diffractive opticalelements for diffractive data storage reading.

FIG. 5 a through FIG. 5 c illustrate a class of deformable cantileverbeam micromirrors (CBM). A description of how to make CBMs is describedin the paper by Hubert Lakner et al. entitled “Micromirrors for directwriting systems and scanners”, Fraunhofer Institute of MicroelectronicsCircuits and Systems, Grenzstrasse 28, D-01109 Dresden, Germany. FIG. 5a shows a top view of the pyramidal element 10 comprising mobile parts12, 13 crossing a silicon substrate 21 having at its corners four posts14 attached to the substrate 21. FIG. 5 a shows a side view of thepyramidal element 10 having the four mirror segments 23 supported by thesupporting posts 14 on the silicon substrate 21. The freestanding mirrorelements 23 are suspended by the supporting posts 14 over the air gap 29with an underlying address electrode 22. The address electrode 22 emitsan electric field which will move the mirror elements 23. An appliedforce produce by electric power source 20 applied between the mirrorelements 23 and the address electrode 22 causes the mirror elements 23to deform into the air gap 29 due to the acting electrical forces. Thevoltage of the electric power source 20 is controlled by a driver thatcan be connected to a controller, such as a microprocessor or computer.The dotted line 26 in FIG. 5 b indicates the position of the mirrorelements 23 when they are not activated. The mirror elements can movedownward until they contact the stopper 27 which prevents them fromcontacting the address electrode 22. FIG. 5 c shows the principal ofphase modulation by the pyramidal element 10. The incoming light 33interacts with the deformed micromirror elements 23 to produce the phaseprofile 31 as shown in the formula 34. As the voltage of the powersource 20 varies, the mirror elements 23 open and close like a “dynamicflower”.

FIGS. 6 a through 6 c represent a micromirror configured as a pistonelement 40 realized by microlithography on a substrate 45. A mirrorplate 48 hinged to four cantilever beams 41 gives rise to a piston-likemotion upon electrical activation of power source 44. The fourcantilever beams (i.e., flexible arms) 41 are supported by the foursupport posts 42. It therefore allows for a pixelwise adjustment of thephase of the incident light. FIG. 6 a shows a top view of the pistonelement 40. The substrate 45 has four support posts 42. The flexiblearms 41 move the piston with a spring effect. The power source 44creates a voltage difference between the mirror 48 and the addresselectrode 46. This constitutes the actuator with the flexible arm 41,pivoting the mirror 48 on the supporting post 42. The holes 43 areimplemented during the manufacturing process to allow handling of thecomponent in the manufacturing process.

The pistons 41 operate just like a set of micro lift platforms going upand down staying parallel to themselves. As shown in FIG. 6 c, theincoming wavefront 54 is modulated as determined by the driver voltage44. The pistons 41 are controlled by voltage applied to the addresselectrodes 46. This voltage 44 is controlled by a driver that can beconnected to a computer. With reference to FIG. 6 c, the phasemodulation resulting from the dynamic positioning of the piston element40 is described. The low level of displacement 51 a puts the mirror 48in position 53. The high level of displacement 51 b puts the mirror 48in the position 52.

FIG. 7 a shows an SEM (scanning electronic microscope) photograph of anarray 62 of pyramidal elements 10. FIG. 7 b shows an SEM photograph ofan array 64 of piston elements 40. The motion of the pyramidal elements10 or piston elements 40 in the arrays of FIG. 7 a and FIG. 7 b,respectively, is described as follows. The moving up and down of theelements controls the array of pyramidal elements or piston elementsrepresented in FIGS. 7 a, 7 b, respectively and gives a succession ofdiffractive pattern that can be programmed in a given range, by acomputer. Each, pattern implements an optical function. In other words,the laser light reflected by a piston in a low position will interferewith the light reflected by a piston in a high position that willinterfere with the light reflected by their piston neighbors in the upor down position and so on with all the pistons. With appropriatecalculation from the theory of diffraction, it is thus possible to shapea wavefront (in a limited range) to fulfill application needs. Theprocess is satisfied only when the size of the elements 10, 40 aresufficiently small compared to the wavelength (in the micron andsubmicron range).

FIG. 8 illustrates the principal of using dynamic diffractive optics toimplement addressing of a read beam. A laser beam 50 is shaped byreflection off of a plurality of micro mirrors 54 producing anaddressing beam 52 at a predetermined angle. The micro mirrors 54 may bethe pyramidal elements 10 or piston elements 40 previously described.

The process of angular addressing shown in FIG. 8 can be explained asfollows. A static grating (DOE) will diffract a laser beam and changethe propagation direction of this laser beam depending of the gratingcharacteristics, such as the spacing between the grating lines. Thedirection of the laser beam in a static grating can be changed bychanging the characteristic of the grating. An angular modification willcome from a modification of the spacing of the grating featuresassociates with a specified wavelength range.

In contrast to the static grating, in the dynamic DOE 54, the gratingprofile changes through computer commands altering the micro shape ofthe DOE 54. This has the same effect as changing one static DOE toanother static DOE would have. Thus, the micro mirror elements 54 moveby an electric command without actually moving any device Oust the microsolid state actuator will move). This micro movement changes thedirection of the laser beam wavefront. A DDOE thus replaces a set ofstatic mirror (every one having a specific angle purpose) or a rotationscanning mirror, by one solid state device, lowering the volume,lowering the cost, and allowing for mass production.

FIG. 9 shows a solid state reading system 100 comprising a DDOE array120 comprising diffractive optical elements 10, 40, an acousto-opticsystem 110, a storage medium 130, and a CCD (charge couple device) 140.The computer 150 is configured to control the DDOE array 120, theacousto-optic and amplification system 110 and the CCD device 140. TheDDOE array 120 operates by the diffraction principles illustrated inFIG. 8 using the mirror elements of FIG. 5 and FIG. 6 configured asarrays which is illustrated in FIG. 7. In one embodiment, the storagememory 130 is configured having the storage matrix 8 shown in FIG. 2. Inalternate embodiments, the diffractive patterns could be stored in atape or disc form, as well as the sheet matrix form. The acousto-optic(AO) device 110 directs a laser beam at an angle to the micromirrorarray 120. FIG. 10 shows an individual acousto-optic device 110according to the present invention. When acousto-optical crystals aresubjected to stress, especially by means of a transducer usuallyconsisting of a piezoelectric crystal, they modify the angle ofdiffraction of the light and, in general, of the electromagnetic wavewhich passes through them. In order to modify the value of thediffraction angle of the emerging beam. All that is therefore requiredis to modify the actuating frequency of the piezoelectric transducer.

Thus, as shown in FIG. 10, the variations in orientation along OX and OY(referring to the rectilinear co-ordinates of FIG. 2) of the incidentread beam 125 emanating from the low-power laser 115 are obtained bysubjecting this beam to two acoustooptic components 121, 122.Consequently it may be understood that, by varying the vibrationfrequency of the piezoelectric crystal associated with the acoustoopticcomponent(s), it becomes possible to modify, very rapidly, the desiredorientation of the grating within the rows and columns of thedata-carrying matrix 130. The limiting factor then becomes the dynamicdiffractive optical elements which act on the angle of incidence of theread beam at the matrix 130.

The wavefront from the laser 115 of the acousto-optic system 110 is thusdirected at an angle to the DDOE mirror array 120. The DDOE array, asexplained above, uses MEMS technology. But, it should be mentioned thatMEMS technology has also been used in the prior art for angularlyrouting a laser beam only with a reflective effect and not with adiffractive effect. In the case of pure reflection, the diffractioneffect is actually considered to be noise. It should be noted that theelements 10, 40 are being used herein in the arrays 62, 64 asdiffractive elements and not as mirrors for reflection.

FIG. 9 shows the system 100 uses a plurality of DDOE elements 120operating by phase shifting. The acousto-optic device 110 gives a fastangular XY beam for scanning the storage medium 130. The DDOE ispartitioned into cells. Every partition of the DDOE is addressed by thescanning (XY axis) of the beam coming from the AO. The AO is a doubledevice implementing XY scanning. This means that the scanners areorthogonally mounted with one scanning the vertical direction (y) andthe other scanning the horizontal direction (x).

Every part of the DDOE array 120 is programmed to address the matrix 130at a given time on the XY axis. By this method, all of the points on thematrix 130 can be addressed. That is, all the XY coordinated can bereached. The angular multiplexing is taken into account sincecombination of the AO 110 and the DDOE array 120 can also address withsuitable programming every point of the storage medium 130 at aplurality of beam angles.

FIG. 11 shows the coherent light beam 220 input to the acousto-opticdevice (AOD) 110 which is capable of deflecting the light beam 220through a range of angles. Three possible deflections are shown of thelight beam 220 at different angles to create the light beams 222, 224,226. A rapid response time is achievable with the AOD 110. The maximumangle deflected by the AOD 110 is approximately 4 degrees. However, theangular range typically necessary for addressing the memory 130 isapproximately 30 degrees. An array of diffractive optical elements(DDOE) 120 compensates for the limited addressing angle range of the AOD110. The size of a diffractive optical element of the array 120 has tobe at least one square millimeter to process the complete coherent lightbeam 220.

The number of actual DDOE elements in the DDOE array 120 (each elementhaving 200 dynamic cells of 5×5 microns) will depend on the angularrange targeted. For instance, to reach the 30° angular range it isnecessary to use 8 DDOEs with a geometrical distance (d) between DDOEarray 120 and the AO 110 of 20 centimeters. The number of packets ofdata that can be recorded will depend of the number of DDOEs. Thestorage capacity that can be addressed will depend also on the DDOE cellsize. It will also depend on the distance between the AO 110 and theDDOE array 120. So a compromise is selected for every application withinthe above-mentioned range. The angular control is realized bycontrolling the voltage applied to the AO 110. For a given voltage valuea specific angle will induce a specific addressing angle. This specificangle will be the output beam angle of the AO 110. The beam coming fromthe AO 110 is then routed to the DDOE 120 which will in turn diffractthe beam toward the memory 130 to address a selected packet therein. TheDDOE 120 has an angular addressing range between 4 and 5 degree.

The output beam from DDOE 110 is diffracted. This diffractive process isfurther explained in reference to FIG. 12. FIG. 12 represents arectangular profile grating that can be obtained according to theequation:sin(β)−sin(α)=λ/a

where “λ” is the wavelength and “a” is the grating period as shown inFIG. 13.

FIG. 13 represents an enlarged view of the grating of the memory 130.The minimum feature size of a cell 310 is 5 microns. This represents astatic grating. Every cell (piston) has a 5×5 micron dimension. Tomodify the grating step it is necessary to activate selected activatorsand not activate other ones. By changing the grating step thediffraction angle will change according to the equation associated withFIG. 12. This dynamic step programming allows 5 degrees of angularaddressing angle to be achieved. To increase this range the DDOE is usedas shown in FIG. 14.

FIG. 14 illustrates the operation of achieving the targeted angularrange for the DDOE. It is shown that the DDOE plane for every DDOE hasto be located in reference to the vertical axis. The possible values ofthis angle θ are displayed in the table of FIG. 15. These values werecalculated with the equation introduces in FIG. 12.

Referring to FIG. 15, the first column gives the reference number of thediffractive optical element (DDOE). The second column gives the value ofθ for every DDOE. The third column give the minimal diffracted angle.The fourth column represents the maximum diffracted angle. For everyDDOE the diffracted beam angle is between a minimum and maximumdepending on the dynamic grating step value. From this table, it can beseen that the extreme values are 2 degrees and 44 degrees. Thus, byselecting one of these angles of the table of FIG. 15, the range of 30degrees required for the present invention is satisfied.

The present invention is not to be limited in scope by the specificembodiments described herein. Indeed, various modifications of thepresent invention, in addition to those described herein, will beapparent to those of skill in the art from the foregoing description andaccompanying drawings. Thus, such modifications are intended to fallwithin the scope of the appended claims.

1. An apparatus for reading information from a diffractive optics memoryhaving stored therein information located at a plurality of points ofsaid memory and at a plurality of angles at each one of said points soas to form a plurality of packets of information at each one of saidpoints, comprising: a laser generating said coherent light beam; aplurality of dynamic diffractive optical elements configured to shapeand angular direct a wavefront of a coherent light beam to said memoryat one of said angles of one of said points to reconstruct one of saidpackets of information; and an acousto-optic device for deflectingtoward said dynamic diffractive optical elements said wavefront at adeflection angle with respect to a plane formed by said diffractiveoptical elements wherein the dynamic diffractive optical elements arearranged with different angles with respect to the diffractive opticsmemory and the acousto-optic device so that the dynamic diffractiveoptical elements provide respective minimum and maximum diffractiveangles and said wavefront is shaped by phase shifting of saiddiffractive optical elements.
 2. The apparatus of claim 1, furthercomprising: a detector array comprising a plurality of cells receiving aportion of said wavefront deflected by said diffractive optical elementsand deflected by said memory.
 3. The apparatus of claim 2, wherein saiddetector array is a CCD detector array.
 4. The apparatus of claim 1,wherein said laser is a low powered laser configured to produce saidcoherent light beam.
 5. An apparatus for reading information from adiffractive optics memory having stored therein information located at aplurality of points of said memory and at a plurality of angles at eachone of said points so as to form a plurality of packets of informationat each one of said points, comprising: a laser generating said coherentlight beam; a plurality of dynamic diffractive optical elementsconfigured to shape and angular direct a wavefront of a coherent lightbeam to said memory at one of said angles of one of said points toreconstruct one of said packets of information; and an acousto-opticdevice for deflecting toward said dynamic diffractive optical elementssaid wavefront at a deflection angle with respect to a plane formed bysaid diffractive optical elements wherein the dynamic diffractiveoptical elements are arranged with different angles with respect to thediffractive optics memory and the acousto-optic device so that thedynamic diffractive optical elements provide respective minimum andmaximum diffractive angles, wherein each of said diffractive opticalelements comprises a pyramidal element.
 6. An apparatus for readinginformation from a diffractive optics memory having stored thereininformation located at a plurality of points of said memory and at aplurality of angles at each one of said points so as to form a pluralityof packets of information at each one of said points, comprising: alaser generating said coherent light beam; a plurality of dynamicdiffractive optical elements configured to shape and angular direct awavefront of a coherent light beam to said memory at one of said anglesof one of said points to reconstruct one of said packets of information;and an acousto-optic device for deflecting toward said dynamicdiffractive optical elements said wavefront at a deflection angle withrespect to a plane formed by said diffractive optical elements whereinthe dynamic diffractive optical elements are arranged with differentangles with respect to the diffractive optics memory and theacousto-optic device so that the dynamic diffractive optical elementsprovide respective minimum and maximum diffractive angles, wherein eachof said diffractive optical elements comprises a piston element.
 7. Anapparatus for reading information from a diffractive optics memoryhaving stored therein information located at a plurality of points ofsaid memory and at a plurality of angles at each one of said points soas to form a plurality of packets of information at each one of saidpoints: a laser generating said coherent light beam; a plurality ofdynamic diffractive optical elements configured to shape and anglardirect a wavefront of a coherent light beam to said memory at one ofsaid angles of one of said points to reconstruct one of said packets ofinformation; an acousto-optic device for deflecting toward said dynamicdiffractive optical elements said wavefront at a deflection angle withrespect to a plane formed by said diffractive optical elements whereinthe dynamic diffractive optical elements are arranged with differentangles with respect to the diffractive optics memory and theacousto-optic device so that the dynamic diffractive optical elementsprovide respective minimum and maximum diffractive angles, and acomputer configured to program said diffractive optical elements andsaid acousto-optic device so as to address said memory at one of saidpoints and one of said angles to reconstruct one of said packets.
 8. Anapparatus for reading information from a diffractive optics memoryhaving stored therein information located at a plurality of points ofsaid memory and at a plurality of angles at each one of said points soas to form a plurality of packets of information at each one of saidpoints, comprising: a laser generating said coherent light beam; aplurality of dynamic diffractive optical elements configured to shapeand anglar direct a wavefront of a coherent light beam to said memory atone of said angles of one of said points to reconstruct one of saidpackets of information; and an acousto-optic device for deflectingtoward said dynamic diffractive optical elements said wavefront at adeflection angle with respect to a plane formed by said diffractiveoptical elements, wherein the dynamic diffractive optical elements arearranged with different angles with respect to the diffractive opticsmemory and the acousto-optic device so that the dynamic diffractiveoptical elements provide respective minimum and maximum diffractiveangles and each of said plurality of points stores one or more of saidpackets of information.
 9. A method for reading information from adiffractive optics memory having stored therein information in aplurality of packets, each one of said packets defined by one of aplurality of points and one of a plurality of angles of said memory,comprising the steps of: generating a coherent light beam; diffractivethe wavefront of said coherent light beam with a plurality of dynamicdiffractive optical elements so that said wavefront addresses saidmemory at one of said points and at one of said angles to reconstructsaid one of said packets; and directing said light beam towards saiddynamic diffractive optical elements at a deflection angle using anacousto-optic device, said dynamic diffractive optical elements beingarranged with different angles with respect to the diffractive opticsmemory and the acousto-optic device so that the dynamic diffractiveoptical elements provide respective minimum and maximum diffractiveangles, wherein said wavefront is shaped by phase shifting of saiddynamic diffractive optical elements.
 10. The method for readinginformation of claim 9, further comprising the step of: directing saidlight beam towards said dynamic diffractive optical elements at adeflection angle using an acousto-optic device.
 11. The method of claim9, further comprising the step of: programming said diffractive opticalelements so as to address said memory at one of said points and one ofsaid angles to reconstruct one of said packets.
 12. The method of claim9, further comprising the step of: detecting with a detector arraycomprising a plurality of cells a portion of said light beam diffractedby said memory.
 13. The method of claim 9, further comprising the stepof: generating said coherent light beam from a laser.
 14. The method ofclaim 12, wherein said detector array is a CCD detector array.
 15. Amethod for reading information from a diffractive optics memory havingstored therein information in a plurality of packets, each one of saidpackets defined by one of a plurality of points and one of a pluralityof angles of said memory, comprising the steps of: generating a coherentlight beam; diffractive the wavefront of said coherent light beam with aplurality of dynamic diffractive optical elements so that said wavefrontaddresses said memory at one of said points and at one of said angles toreconstruct said one of said packets; and directing said light beamtowards said dynamic diffractive optical elements at a deflection angleusing an acousto-optic device, said dynamic diffractive optical elementsbeing arranged with different angles with respect to the diffractiveoptics memory and the acousto-optic device so that the dynamicdiffractive optical elements provide respective minimum and maximumdiffractive angles, wherein each one of said dynamic diffractive opticalelements comprises a pyramidal element.
 16. A method for readinginformation from a diffractive optics memory having stored thereininformation in a plurality of packets, each one of said packets definedby one of a plurality of points and one of a plurality of angles of saidmemory, comprising the steps of: generating a coherent light beam;diffractive the wavefront of said coherent light beam with a pluralityof dynamic diffractive optical elements so that said wavefront addressessaid memory at one of said points and at one of said angles toreconstruct said one of said packets; and directing said light beamtowards said dynamic diffractive optical elements at a deflection angleusing an acousto-optic device, said dynamic diffractive optical elementsbeing arranged with different angles with respect to the diffractiveoptics memory and the acousto-optic device so that the dynamicdiffractive optical elements provide respective minimum and maximumdiffractive angles, wherein each one of said dynamic diffractive opticalelements comprises a piston element.
 17. A method for readinginformation from a diffractive optics memory having stored thereininformation in a plurality of packets, each one of said packets definedby one of a plurality of points and one of a plurality of angles of saidmemory, comprising the steps of: generating a coherent light beam;diffractive the wavefront of said coherent light beam with a pluralityof dynamic diffractive optical elements so that said wavefront addressessaid memory at one of said points and at one of said angles toreconstruct said one of said packets; and directing said light beamtowards said dynamic diffractive optical elements at a deflection angleusing an acousto-optic device, said dynamic diffractive optical elementsbeing arranged with different angles with respect to the diffractiveoptics memory and the acousto-optic device so that the dynamicdiffractive optical elements provide respective minimum and maximumdiffractive angles, wherein each of said plurality of points stores aplurality of packets and each packet comprises a sub-holographic image.